Methods of making polycrystalline diamond and/or cubic boron nitride (CBN) cutting tools are well known. Such methods may include placing within a machined graphite heater tube, a substrate such as a metal carbide shank and an unsintered mass of abrasive crystalline diamond or CBN particles adjacent the substrate. Additional salt plugs, salt liners, and graphite end plugs may also be placed within the heater tube, which plugs or liners substantially remove cavities or voids within the heater tube. Metal end caps may be placed adjacent each end of the heater tube to provide electrical contacts for applying a current to the heater tube.
The resulting heater tube assembly may then be placed within a correspondingly shaped cavity of a high-pressure cell comprised of salt or talc, which cell is in turn placed within a high-pressure press. The entire assembly is compressed under high pressure, and an electrical current is applied to produce sufficiently high pressures and temperatures to effect intercrystalline bonding between adjacent grains of the abrasive particles and joining of the sintered particles to the metal carbide substrate. U.S. Pat. No. 3,767,371 of Oct. 23, 1973, U.S. Pat. No. 3,745,623 of Jul. 17, 1973, and U.S. Pat. No. 5,512,235 of Apr. 30, 1996, which are hereby incorporated by reference herein, show various examples of prior art methods in which heater tubes are employed in high temperature and pressure systems to produce diamond or CBN polycrystalline compacts which are bonded to a metal carbide substrate. The resulting diamond tipped or CBN tipped substrates are then integrated into a cutting tool used for drilling, milling, grinding, sawing or any other high pressure and temperature application.
Prior art heater tubes must be constructed within precise tolerances with respect to their dimensions to ensure that the variously inserted substrates, particle holding salt dishes, salt liners, and graphite plugs fill substantially all of the space within the heater tube. In the prior art, to produce heater tubes with the required dimensions, the heater tubes are machined from extruded graphite. However, extruded graphite tends to be relatively brittle; as a result, some heater tubes will crack during the preparation of the heater tube and/or when high pressures are applied as described above. Such cracks may disrupt the uniform generation of heat from the applied electrical current. In these cases, the resulting diamond or CBN tipped tool may be defective.
Thus there exists a need for a new heater tube which can be readily substituted into an existing processes for making diamond or CBN tipped tools and which is less likely to produce defective diamond or CBN tipped tools.